The present invention relates to a transfer device for transferring a visible image such as a toner image from a visible image carrying belt such as a photosensitive member or an intermediate transfer belt onto a transfer member such as a transfer sheet, and to an image forming apparatus such as a facsimile, a printer or a copying machine provided with the transfer device.
FIG. 4 illustrates a conventionally known transfer device of this type. In FIG. 4, a transfer device 100 comprises an intermediate transfer belt 101 serving as a visible image carrying belt, a secondary transfer bias roller 102 serving as a transfer current applying member, a secondary transfer bias power source, not illustrated, for applying a secondary transfer bias to the secondary transfer bias roller 102, a secondary transfer facing roller 103, a drive roller 104, a tension roller 105, a primary transfer bias roller 106 and a primary transfer grounding roller 107.
The intermediate transfer belt 101 is driven to be rotated in a direction indicated by an arrow B via the drive roller 104 while being stretched across the secondary transfer facing roller 103, the drive roller 104, the tension roller 105, the primary transfer bias roller 106 and the primary transfer grounding roller 107. A portion of the intermediate transfer belt 101, positioned between the primary transfer bias roller 106 and the primary transfer grounding roller 107 is urged toward a photosensitive drum 1 in an image forming apparatus via the rollers 106 and 107, to be positively brought into contact with the photosensitive drum 1, and thus, it forms a primary transfer position.
At the primary transfer position, a primary transfer current is applied to the intermediate transfer belt 101 from the primary transfer bias roller 106, thereby forming a primary transfer electric field therebetween. Most of the primary transfer current applied to the intermediate transfer belt 101 is introduced to a ground via the primary transfer grounding roller 107.
The intermediate transfer belt 101 is sandwiched between the secondary transfer bias roller 102 and the secondary transfer facing roller 103, thereby forming a secondary transfer nip as a transfer position. At the secondary transfer nip, a secondary transfer electric field is formed by a secondary transfer bias which has a polarity opposite to that of a toner and is applied from the secondary transfer bias roller 102 to the intermediate transfer belt 101.
The drive roller 104 and the tension roller 105 are brought into contact with the reverse of the intermediate transfer belt 101 downstream and upstream of the secondary transfer nip (hereinafter referred simply to as a nip downstream side and a nip upstream side, respectively), respectively, and thus, introduce a residual electric charge on the intermediate transfer belt 101 to grounds.
A toner image formed on the photosensitive drum 1 is primarily transferred onto the intermediate transfer belt 101 by the effect of the primary transfer electric field or the like when the intermediate transfer belt 101 passes through the primary transfer position as it is driven to be rotated. The intermediate transfer belt 101 having the primarily transferred toner image formed thereon intrudes in the secondary transfer nip as the intermediate transfer belt 101 is transported.
In the meantime, a sheet feeder, not shown, in the image forming apparatus feeds a transfer sheet 10 toward the secondary transfer nip at such a timing that the toner image is superimposed on the transfer sheet 10. The toner image superimposed on the transfer sheet 10 at the secondary transfer nip is secondarily transferred from the intermediate transfer belt 101 onto the transfer sheet 10 by the effect of a nip inner pressure, the secondary transfer electric field or the like.
In the transfer device 100 such configured as described above, in the case where the secondary transfer bias power source is of a type for applying the secondary transfer bias of a predetermined voltage to the secondary transfer bias roller 102, the secondary transfer current flowing from the secondary transfer bias roller 102 to the intermediate transfer belt 101 is undesirably varied when the electric resistance of the intermediate transfer belt 101 is varied according to environmental fluctuations. In this case, if the secondary transfer current is varied, secondary transfer performance becomes unstable, so that a secondary transfer image of a stable quality cannot be obtained.
Therefore, there are generally used secondary transfer bias power sources of a type for making constant the secondary transfer current from the secondary transfer bias roller 102 to the intermediate transfer belt 101 by a constant current control or the like. Such a secondary transfer bias power source can apply a constant secondary transfer current to the intermediate transfer belt 101 irrespective of the fluctuations in electric resistance of the intermediate transfer belt 101, so as to stabilize the secondary transfer performance.
However, in the transfer device 100 illustrated in FIG. 4, even if the secondary transfer performance is stabilized by applying the constant secondary transfer current to the intermediate transfer belt 101, the edge of the secondary transfer image secondarily transferred onto the transfer sheet 10 may often become unclear depending upon the environment of temperature or humidity.
The present inventor has earnestly studied on causes which make the edge of the secondary transfer image unclear. As a result, he or she has found a phenomenon as follows: the secondary transfer current flowing from the secondary transfer bias roller 102 to the intermediate transfer belt 101 is divided into a downstream current A1 which is transmitted downstream of the nip inside the belt so as to flow in the drive roller 104, a nip current A2 which is transmitted in a belt thickness direction inside the nip so as to flow in the secondary transfer facing roller 103, and an upstream current A3 which is transmitted upstream of the nip inside the belt so as to flow in the tension roller 105, as shown in FIG. 5. It has been found that among these currents, the upstream current A3 disperses some of innumerous toners forming the toner image on the intermediate transfer belt 101 before intruding into the secondary transfer nip from an image portion to the surroundings of a non-image portion, thereby generating toner dispersion.
A downstream resistance R1, a nip resistance R2 and an upstream resistance R3 act on the downstream current A1, the nip current A2 and the upstream current A3, respectively. As illustrated in FIG. 6, the downstream resistance R1 is equivalent to the sum of an electric resistance over a length L1 from the secondary transfer nip of the intermediate transfer belt 101 to a contact position with the drive roller 104 and an electric resistance of the drive roller 104; the nip resistance R2 is equivalent to the sum of an electric resistance of the intermediate transfer belt 101 in a thickness direction and an electric resistance of the secondary transfer facing roller 103; and the upstream resistance R3 is equivalent to the sum of an electric resistance over a length L2 from the secondary transfer nip of the intermediate transfer belt 101 to a contact position with the tension roller 105 and an electric resistance of the tension roller 105.
The reason why unclearness of the edge is found or not depending upon the environment is as follows. Namely, variations in electric resistance according to the environmental fluctuations are generated in not only the intermediate transfer belt but also the secondary transfer facing roller 103, the drive roller 104 or the tension roller 105. If at least one of these rollers is varied in electric resistance at a varying rate different from those of the other rollers, a rate occupied by the upstream resistance R3 is varied with respect to the entire resistance R0 which is the sum of the electric resistances R1 to R3. If the environment is changed so as to reduce the rate, the upstream current A3 is naturally increased. When the increase reaches a predetermined value, the toner dispersion adversely proceeds to such an extent as to be visually recognized with ease, and consequently, the dispersed toner is secondarily transferred together with the toner image at the secondary transfer nip, thereby making the edge of the secondary transfer image unclear.
Incidentally, although the description has been given of the transfer device for applying the secondary transfer bias having the polarity opposite to that of the toner to the transfer surface of the intermediate transfer belt 101 in reference to FIGS. 4 to 6, similar toner dispersion may possibly occur also in a transfer device for applying a secondary transfer bias having the same polarity as that of the toner to the reverse of the intermediate transfer belt 110. Not only in the transfer device in which each of the drive roller 104 and the tension roller 105 is grounded but also in a transfer device in which only the tension roller 105 upstream of the nip is grounded, similar toner dispersion may possibly occur if the electric resistances of the secondary transfer facing roller 103 and the tension roller 105 are varied at different varying rates. Moreover, similar toner dispersion may possibly occur not in the transfer device in which the secondary transfer nip is formed but in a transfer device in which a transfer position forming roller in place of the secondary transfer facing roller 103 forms a secondary transfer position in contact with the intermediate transfer belt 101 at a position not facing to the secondary transfer bias roller 102. Additionally, although the explanation has been made on the toner dispersion which may possibly occur when the toner image is secondarily transferred from the intermediate transfer belt 101 onto the transfer sheet 10, similar toner dispersion may possibly occur also when a visible image is transferred from the visible image carrying belt onto the transfer member, for example, when the visible image is primarily transferred from the photosensitive belt onto the transfer member such as the transfer sheet.
It is an object of the present invention to provide a transfer device capable of suppressing unclearness of the edge of a transfer image, caused by an upstream current flowing in a visible image carrying belt upstream of a transfer position such as a nip, and an image forming apparatus provided with the transfer device.
A transfer device according to a first aspect of the present invention comprises a visible image carrying belt carrying a visible image thereon and traveling in a predetermined direction; a transfer current applying member for applying a transfer current in contact with the visible image carrying belt; a transfer position forming member in contact with a surface of the visible image carrying belt on a side opposite to the surface in contact with the transfer current applying member, for introducing a transfer current in a direction of the thickness of the visible image carrying belt so as to form a transfer position between the transfer current applying member and the same; a downstream grounding member disposed in such a manner as to be brought into contact with the visible image carrying belt downstream of the transfer position in the predetermined direction, to be thus electrically grounded; and an upstream grounding member disposed in such a manner as to be brought into contact with the visible image carrying belt upstream of the transfer position in the predetermined direction, to be thus electrically grounded, wherein a visible image on the visible image carrying belt is transferred onto a transfer member to be fed to the transfer position, and a downstream resistance equivalent to the sum of an electric resistance over a length from a contact position with the transfer current applying member to a contact position with the downstream grounding member on the visible image carrying belt and an electric resistance of the downstream grounding member is set to be lower than an upstream resistance equivalent to the sum of an electric resistance over a length from the contact position with the transfer current applying member to a contact position with the upstream grounding member on the visible image carrying belt and an electric resistance of the downstream grounding member irrespective of environmental fluctuations.
In the transfer device, the downstream resistance is set to be lower than the upstream resistance irrespective of the environmental fluctuations, so that the quantity of the upstream current flowing upstream in the belt transporting direction beyond the transfer position is set to be smaller than the quantity of the downstream current flowing downstream in the belt transporting direction beyond the transfer position. With this configuration, the quantity of the upstream current can be excellently reduced so as to suppress the unclearness of the edge of the transfer image caused by the upstream current in comparison with the case where the downstream resistance is set to be equal to or higher than the upstream resistance.
The present inventor has examined the relationship between the quantity of the upstream current flowing from the secondary transfer nip to the tension roller 105 and the unclearness of the edge of the secondary transfer image by the use of a transfer device similar to the transfer device 100 illustrated in FIG. 4. Concretely, the drive roller 104 and the tension roller 105 were replaced in sequence with rollers capable of exhibiting their specific electric resistances, respectively, and then, the quantity of the upstream current A3 was adjusted at various values while adjusting the ratio of the downstream resistance R1 to the upstream resistance R3. An examination was made of the unclearness of the edge of the secondary transfer image when the upstream current A3 of each of the values flowed. The examination result is shown below in Table 1.
In Table 1, xe2x80x9c◯xe2x80x9d designates that no unclearness of the edge was visually recognized; xe2x80x9cxcex94xe2x80x9d designates that the unclearness of the edge was visually recognized in a slight fashion; and xe2x80x9cxc3x97xe2x80x9d designates that the unclearness of the edge was visually recognized with ease. As shown in Table 1, it is found that if the upstream current A3 is set at 3.2 (xcexcA) or lower, the unclearness of the edge of the secondary transfer image caused by the upstream current A3 can be suppressed to such a level as not to be visually recognized.
Thus, a transfer device according to a second aspect of the present invention comprises: a visible image carrying belt carrying a visible image thereon and traveling in a predetermined direction; a transfer current applying member for applying a transfer current in contact with the visible image carrying belt; a transfer position forming member in contact with a surface of the visible image carrying belt on a side opposite to the surface in contact with the transfer current applying member, for introducing a transfer current in a direction of the thickness of the visible image carrying belt so as to form a transfer position between the transfer current applying member and the same; and grounding members in contact with the visible image carrying belt at positions different from that of the transfer position forming member, to be thus electrically grounded, wherein a visible image on the visible image carrying belt is transferred onto a transfer member to be fed to the transfer position, and the quantity of a current flowing upstream of the transfer position in the predetermined direction on the visible image carrying belt with the application of the transfer current is maintained at 3.2 xcexcA or lower irrespective of environmental fluctuations.
In the transfer device, the quantity of the upstream current is maintained at 3.2 xcexcA or lower irrespective of the environmental fluctuations, thereby suppressing the unclearness of the edge of the transfer image caused by the upstream current to such a level as not to be visually recognized.
A transfer device according to a third aspect of the present invention, as related in the second aspect, environmental conditions suitable for use are designated, and the visible image carrying belt, the transfer position forming member and the grounding members each are composed of a material capable of exhibiting resistance varying characteristics in such a manner as to maintain the quantity of the current at 3.2 xcexcA or lower as long as the transfer device is used under the environmental conditions.
In the transfer device, the environmental condition suitable for the use is designated by explicitly describing, for example, the temperature range or humidity range suitable for the use in a seal stuck to the device or an instruction manual. In the meantime, the visible image carrying belt such as an intermediate transfer belt, the transfer position forming member such as a secondary transfer facing roller and the grounding members such as a drive roller and a tension roller can exhibit such resistance varying characteristics as to maintain the quantity of the upstream current at 3.2 xcexcA or lower under the above-described environmental condition. With this configuration, the unclearness of the edge of the transfer image caused by the upstream current can be suppressed to such a level as not to be visually recognized under the designated environmental condition.
A transfer device according to a fourth aspect of the present invention, as related in the second aspect, the visible image carrying belt, the transfer position forming member and the grounding members each are composed of a material capable of exhibiting resistance varying characteristics in such a manner as to maintain the quantity of the current at 3.2 xcexcA or lower within a temperature range from 10xc2x0 C. or higher to 40xc2x0 C. or lower irrespective of temperature fluctuations, or within a humidity range from 15% or more to 80% or less irrespective of humidity fluctuations.
In the transfer device, the unclearness of the edge of the transfer image caused by the upstream current can be suppressed to such a level as not to be visually recognized irrespective of the fluctuations in temperature or the fluctuations in humidity within a general function ensuring temperature range (from 10xc2x0 C. to 40xc2x0 C.) or a general function ensuring humidity range (from 15% to 80%) in the transfer device.
A transfer device according to a fifth aspect of the present invention, as related in the second aspect, there are provided a transfer power source for supplying the transfer current to the transfer current applying member and current detecting means for detecting the quantity of the current, and the transfer power source is configured such that an output current is controlled in such a manner that the quantity of the current detected by the current detecting means becomes 3.2 xcexcA or lower.
In the transfer device, the quantity of the upstream current is maintained at 3.2 xcexcA or lower by controlling the quantity of the upstream current by means of the transfer voltage power source even unless the respective resistance varying characteristics of the visible image carrying belt, transfer position forming member and grounding members are finely adjusted, so that the unclearness of the edge of the transfer image caused by the upstream current can be suppressed to such a level as not to be visually recognized.
An image forming apparatus according to a sixth aspect of the present invention comprises: visible image forming means for forming a visible image on a visible image carrying belt; and a transfer device for transferring the visible image from the visible image carrying belt onto a transfer member, wherein an image is formed by transferring the visible image onto the transfer member, and the transfer device is according to the first, second, third, fourth or fifth aspect.
In the image forming apparatus, the quantity of the upstream current can be excellently reduced so as to suppress the unclearness of the edge of the transfer image caused by the upstream current by the effect similar to that of the transfer device according to the first aspect in the case where the image forming apparatus is provided with the transfer device according to the first aspect in comparison with the case where the downstream resistance is equal to or higher than the upstream resistance inside the transfer device. Furthermore, the unclearness of the edge of the transfer image caused by the upstream current can be suppressed to such a level as not to be visually recognized irrespective of the environmental fluctuations by the effect similar to that of the transfer device according to the second aspect in the case where the image forming apparatus is provided with the transfer device according to the second, third, fourth or fifth aspect. Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.